A conventional aluminum electrolytic capacitor has been arranged as shown in FIG. 8. Namely, in FIG. 8, a capacitor element 1 is impregnated with driving electrolyte and two lead-out lead plates 2a and 2b are drawn from the capacitor element 1. A mouth sealing member 3 for sealing a mouth of a cylindrical metallic casing 9 made of, for example, aluminum and having a bottom is formed by laminating a rubber plate 3a and a resinous plate 3b integrally, while a-pair of metallic rivets 4a made of, for example, aluminum and a pair of metallic rivets 4b made of, for example, aluminum are pierced through the mouth sealing member 3. An external connecting terminal 5a is connected to one end of one of the rivets 4a and the lead-out lead plate 2a is connected to the other end of the one of the rivets 4a by crimping through metallic washers 6a and 7a made of, for example, aluminum. Likewise, an external connecting terminal 5b is connected to one end of one of the rivets 4b and the lead-wire lead plate 2b is connected to the other end of the one of the rivets 4b by crimping through metallic washers 6b and 7b made of, for example, aluminum. The capacitor element 1 and the mouth sealing member 3 provided integrally in this way are accommodated in the metallic casing 9 containing fixing agent 8. The metallic casing 9 is subjected to transverse drawing in the vicinity of the mouth. A distal end of the mouth of the metallic casing 9 is subjected to curling so as to retain the mouth sealing member 3. Meanwhile, a weak spot (not shown) formed by a thin wall portion is provided on the bottom of the metallic casing 9.
In case a voltage higher than its rated voltage is applied to the conventional aluminum electrolytic capacitor, temperature in the metallic casing 9 rises, so that organic solvent forming the driving electrolyte impregnated in the capacitor element 1 is vaporized and hydrogen gas is generated by electrochemical reaction. As a result, internal pressure of the metallic casing 9 accommodating the capacitor element 1 rises. If the internal pressure of the metallic casing 9 rises excessively, the weak portion provided on the bottom of the metallic casing 9 is ruptured, so that the organic solvent gas flows out of the metallic casing 9 through the ruptured weak portion, thereby resulting in prevention of a tremendous explosion of the metallic casing 9.
However, in the above known aluminum electrolytic capacitor, since the driving electrolyte gas spouts out of the metallic casing 9 in misty state, such problems arise that this spouted misty driving electrolyte gas soils interior of an electronic appliance incorporating the known aluminum electrolytic capacitor and is mistaken for smoke due to a fire.
In order to solve these problems of the known aluminum electrolytic capacitor, an electrolytic capacitor with an open circuit mode mechanism as shown in FIGS. 9A and 9B is proposed in, for example, Japanese Utility Model Publication No. 6-39446 (1994). This prior art electrolytic capacitor with the open circuit mode mechanism is arranged as follows. Namely, in FIG. 9A, a capacitor element 12 is accommodated in a metallic casing 11 and a metallic upper lid 14 is hermetically mounted on an upper peripheral edge of a mouth of the metallic casing 11 through an annular elastic packing 13. A pair of first rivets 15 are pierced through the metallic upper lid 14 and each of the first rivets 15 is a composite rivet in which a metallic rivet 15b made of iron or metal other than aluminum is press fitted into an opening of an aluminum hollow rivet 15a so as to be welded to the aluminum hollow rivet 15a. Each of the first rivets 15 not only is hermetically mounted on the metallic upper lid 14 but is electrically insulated from the metallic upper lid 14 by a silicone rubber piece 17 which is compressedly gripped between an insulating member 16 molded by resin and the aluminum hollow rivet 15a.
Meanwhile, an external terminal 18 is welded to an upper end of the metallic rivet 15b. An aluminum foil plate 19 having a weak portion is welded to the aluminum hollow rivet 15a and is fixed, through an aluminum washer 23, to a lead-out lead foil plate 22 from the capacitor element 12 by a second rivet 21 mounted on a fixing plate 20 molded by resin so as to be connected to the lead-out lead foil plate 22.
In the prior art electrolytic capacitor with the open circuit mode mechanism as shown in FIG. 9A, in case pressure in the metallic casing 11 rises due to a malfunction in the prior art electrolytic capacitor as shown in FIG. 9B, the metallic upper lid 14 is deformed greatly to swell out upwardly upon rise of the pressure in the metallic casing 11. Therefore, in response to this upward deformation of the metallic upper lid 14, the first rivet 15 which not only is hermetically mounted on the metallic upper lid 14 but is electrically insulated from the metallic upper lid 14 by the compressed silicone rubber piece 17 is also deformed upwardly, so that the aluminum foil plate 19 welded to the aluminum hollow rivet 15a of the first rivet 15 is pulled upwardly and thus, is cut off at the weak portion. As a result, connection between the aluminum foil plate 19 and the lead-out lead foil plate 22 from the capacitor element 22 is cut and thus, an electric circuit of the prior art electrolytic capacitor is broken.
However, in the prior art electrolytic capacitor with the open circuit mode mechanism as shown in FIGS. 9A and 9B, as many as six components, i.e., the metallic rivet 15b and the aluminum hollow rivet 15a of the first rivet 15, the aluminum foil plate 19, the second rivet 21, the aluminum washer 23 and the lead-out lead foil plate 22 are provided in a flow path of electric current from the external terminal 18 to the capacitor element 12, structure of the prior art electrolytic capacitor is made complicated and the number of its assembly steps also increases. Meanwhile, the prior art electrolytic capacitor has such a disadvantage that the number of connections of the six components reaches as many as five, thereby resulting in poor reliability of connections of the components.
Meanwhile, when the open circuit mode mechanism is actuated, the metallic upper lid 14 is deformed so as to swell out upwardly as shown in FIG. 9B, interval between a pair of the external terminals 18 is increased. Therefore, in case the external terminals 18 are restricted by holes of a printed circuit board or the like, such inconveniences are incurred that the open circuit mode mechanism malfunctions and the printed circuit board is damaged by the external terminals 18 upon increase of interval between the external terminals 18.
Furthermore, since the aluminum foil plate 19 having the weak portion of small cross-sectional area for reducing its mechanical strength is provided in the course of the electric circuit for passing large AC therethrough from the external terminal 18 to the capacitor element 12, electric resistance rises at the weak portion and thus, the weak portion is heated by the AC. As a result, the prior art electrolytic capacitor has such a disadvantage that ripple current (AC) capacity, one of basic features of the electrolytic capacitor is sacrificed.